1. Field of the Invention
The present invention relates to a heat treatment process for a titanium alloy which results in an end product having significantly improved fatigue qualities. The method is of particular benefit to medical prostheses, for example, joint replacement components, but may be of benefit in any environment requiring high strength, light weight components subjected to elevated, intermittent stresses.
The process calls for temporary diffusion into the metal alloy of a fugitive alloying element to promote a phase transformation in the metal. Hydrogen is of particular interest because it has significant effects on titanium alloys and can be readily removed from the metal after treatment. The alloy is then subjected to a controlled eutectoid transformation after which the fugitive alloying element is removed from the metal.
2. Description of the Prior Art
Hydrogen has previously been used to facilitate the workability of titanium and its alloys. It has been used to embrittle titanium to facilitate its comminution by mechanical means to form titanium metal powders. In such techniques hydrogen is diffused into the titanium at elevated temperatures, the metal is cooled and brittle titanium hydride formed. The brittle material is then fractured to form a powder. The powder may then have the hydrogen removed or a compact may be formed of the hydrided material which is then dehydrided. This process has been described in U.S. Pat. No. 4,219,357 to Yolton et al.
Hydrogen also has the effect of increasing the high temperature ductility of titanium alloys. This characteristic has been used to facilitate the hot working of titanium alloys. Hydrogen is introduced to the alloy which is then subjected to high temperature forming techniques such as forging. The presence of hydrogen allows significantly more deformation of the metal without cracking or other detrimental effects, as related in U.S. Pat. No. 2,892,742 to Zwicker et al.
Clearly our invention does not deal with enhancing the workability of titanium or its alloys, as in the above example. We focus on creating a microstructure with optimum resulting mechanical properties. Such approaches have been followed by others before us: hydrogen has also been used as a temporary alloying element in attempts to alter the microstructure and mechanical properties of titanium alloys. In such applications, hydrogen is diffused into the titanium alloys, the alloys cooled to room temperature and then heated to remove the hydrogen. The effect of the temperature of introducing and removing the hydrogen on the structure and properties of titanium alloys was investigated by W. R. Kerr et al, as part of a larger study, and their findings published in an article entitled: "Hydrogen as an Alloying Element in Titanium (Hydrovac),"Titanium '80 Science and Technology (1980), pp. 2477-2486. In this study, some fundamentals of the so called temporary hydrogen alloying technique were analyzed. The study did not result in an optimum process. A first attempt thereto resulted in the disclosure of Smickley et al in U.S. Pat. No. 4,505,764. They disclose a process of treating a cast titanium alloy (e.g. Ti-6Al-4V) for the purpose of refining its microstructure. The casting is heated to a treatment temperature near, but below, the beta transformation temperature. A solute material, such as hydrogen, in the range of 0.2% to 5% by weight is diffused into the casting at the aforesaid treatment temperature below the beta transformation temperature. A refined structure is created next largely as the result of the reverse transformation and not necessarily as a result of a eutectoid transformation. Subsequently, the metal is allowed to cool to room temperature.
This process is inflexible in that the article must be maintained at high temperature until the last step is performed, i.e. the hydrogen removed. Our invention, in contrast, teaches now to process the article without having to be concerned about thermal cracking resulting from low temperatures during or between any of the steps of the process. Furthermore, the Smickley process can be highly inefficient in view of the very long transformation times needed with the amount of hydrogen that is indiffused uncontrollably during their process. Again, we have solved this drawback by inventing processing conditions yielding optimized hydrogen concentrations.
Levin et al, in U.S. Pat. No. 4,612,066, disclose a process for fabricating titanium alloy components which subsequent to being cast are first transformed to a martensitic structure. Similar processes have been disclosed by Levin et al in U.S. Pat. No. 4,655,855 for refining the microstructure of titanium alloy powder compacts and by Vogt et al in U.S. Pat. No. 4,680,063 for fabricating forged titanium alloy components.
Such a martensitic structure is obtained by rapidly cooling the article. Having the martensitic structure is essential to their invention, since the subsequent steps rely upon the presence of this structure to produce a microstructural refinement. In our invention we exclude the possibility of any martensite. Since phase transformation from a starting martensitic structure, as in Levin et al, or from a starting beta structure, as in our invention, are fundamentally different, Levin's disclosures are materially unimportant with respect to our invention.